Semiconductor lasers are widely used in applications such as optical communications. The edge emitting laser diode is a semiconductor laser that emits light from a plane which is a continuation of the p-n junction of the diode. Cleaved surfaces at the ends of the diode act as mirrors which together define an optical cavity. Optical feedback provided by the cleaved mirrors creates a resonance of the emitted light that results in lasing.
The vertical cavity surface emitting laser (VCSEL) is another type of semiconductor laser in which the optical cavity is normal to the p-n junction of the semiconductor wafer from which it was fabricated. Ordinarily VCSELs are manufactured with many layers of semiconductor material deposited upon the substrate. The VCSEL includes highly reflective optical mirrors above and below the active layer which, in contrast to the edge emitting laser, enable laser output normal to the surface of the wafer.
VCSELs are preferred over edge-emitting devices for a number of applications. Since they emit vertically and the beam is more symmetric and less divergent, coupling VCSELs to fiber or to other optical devices is easier in many cases. Typically a low-cost ball lens may be used rather than expensive aspheres. In addition, VCSELs are fabricated into completed lasers at the wafer level, so fabrication and testing are relatively inexpensive. These properties, combined with the small size of the VCSEL that allows high speed operation at low currents, make them desirable for lower-cost data communications transceivers.
Because of their complexity, however, existing processes for manufacturing VCSELs do not always yield devices with consistent characteristics. The process involves hundreds of layers that depend on numerous parameters including, but not limited to, doping concentration, substrate temperature, material sources, and growth rate. These process parameters compound the manufacturing difficulty already well understood in the semiconductor field where fluctuations on the order of 50-100% are not uncommon. In the case of silicon technology, designers typically use ratios of values to minimize the effect of process variations. Unfortunately, in the case of discrete lasers, there is no suitable existing way to compensate for process variations within the device. The result is that the burden is placed on the higher level assemblies to compensate for device variations, adding complexity and cost.
In the case of data communications, for example, the output power of the transmitter is ordinarily restricted to a specified range. In practice, either the total optical subassembly slope variation falls within specification, or the drive circuit must compensate by driving low slopes with higher currents and higher slopes with lower currents. The drawback with varying the drive currents, however, is that high speed performance varies, affecting the overall product consistency and yield.
Accordingly, a process would be desirable that produces lasers with highly consistent slope efficiencies on a wafer to wafer basis. Slope efficiency, also referred to as external efficiency, or slope, generally refers to the product of the internal efficiency and the optical efficiency. The internal efficiency is the fraction of electrons that are converted to photons, and the optical efficiency is the fraction of photons that are transmitted out of the laser. Since internal efficiency is difficult to precisely control because of the complexity of semiconductor processes, those skilled in the art would prefer a process that enables the tuning of the slope efficiency of the laser by altering the optical efficiency, which is directly related to the transmission and reflectivity of the laser, to compensate for process variations in a relatively simple and cost effective manner.
Some prior art lasers have been fabricated with a non-quarter wavelength layer of optically transparent material that had the side effect of changing the slope. An example of such a prior art VCSEL with a non-quarter wavelength layer has the specification shown in FIG. 12. However, the prior art process changed the slope of the laser in a fixed manner that generally did not take into account wafer to wafer variations. Therefore, any wafer to wafer variations upon application of the fixed layer led to the same variations in the final products. Those skilled in the art would prefer a process that enables predictable tuning during fabrication to achieve lasers having consistent slopes on a wafer to wafer basis.